Remove unused accelerated compositing settings.

The following are removed from WebSettings (and a couple unused
leftovers from Settings.in also):
setAcceleratedCompositingForFixedPositionEnabled
setAcceleratedCompositingForFixedRootBackgroundEnabled
setAcceleratedCompositingForOverflowScrollEnabled
setCompositedScrollingForFramesEnabled

R=abarth
BUG=365851

Review URL: https://codereview.chromium.org/484893002

git-svn-id: svn://svn.chromium.org/blink/trunk@180605 bbb929c8-8fbe-4397-9dbb-9b2b20218538